The New European Digital Video Broadcast (DVB) Standard
-------------------------------------------------------
Markus Kuhn -- 1996-07-08
This text gives a brief introduction into the technology behind the
new DVB/MPEG-2 TV broadcasting system. The latest version of this
text can always be found at .
Use a monospaced font to display it.
Special thanks to Hanno Basse for his many very valuable
comments and contributions to this text, as well as to Didier Aernoudt
for supplying additional information. Errors
in this text are of course my fault. Any comments, additions, and
corrections to this brief overview are very welcome!
[The market situation description of this text is at the moment at some
places not very up to date and describes the situation in early 1996,
but the described standards and technical details are still unchanged.
Markus -- 1996-11-16]
Contents
--------
1 Introduction
2 Building blocks
2.1 MPEG-2 source encoding
2.1.1 Handling 16:9 images on 4:3 screens
2.2 Modem standards
2.2.1 Satellite transmission (DVB-S)
2.2.2 Cable transmission (DVB-C)
2.2.3 Community antenna installations (DVB-CS)
2.2.4 Terrestrial transmission (DVB-T)
2.2.5 Compatibility
2.3 System information and Electronic program guide (DVB-SI)
2.4 Teletext
2.5 Subtitling
2.6 Conditional access control (pay-TV encryption)
2.6.1 Common Interface (DVB-CI)
2.6.2 SimulCrypt
3 Market overview and system introduction schedule
4 Hints for DVB receiver buyers
5 Literature and sources for more information
1 Introduction
The next generation of TV signal broadcast standards is based on
digital data compression and digital data transmission. This provides
both higher image quality and better bandwidth utilization than
classic analog color TV broadcasting standards such as PAL, NTSC, and
SECAM.
In January 1995, the Digital Video Broadcasting (DVB) project
organized by the European Broadcasting Union (EBU) has published a set
of formal standards which define the new Digital Video Broadcast
system. These DVB standards are the technical basis for implementing
digital TV transmission in Europe, Asia, Australia, and many other
regions of the world starting in 1996. DVB seems to be the best
candidate for a single global digital TV broadcasting standard. Only
the U.S. plan their own terrestrial HDTV standard which is also based
on MPEG-2 but they use a modem, an audio encoder, and an electronic
program guide incompatible with DVB standards.
Current DVB standards describe digital TV transmission over satellite
and cable; modulation standards for terrestrial broadcast are under
preparation. DVB standards cover the system design and the modem
standards for high bandwidth video data transmission as well as
several auxiliary functions like teletext, electronic program guides,
and conditional access. The compression technique used by the DVB
system is the ISO MPEG-2 algorithm.
The following brief description of the new DVB standards originated as
my notes from a talk about the topic given by C. Schaaf from German
Telekom, FTZ Darmstadt, at the INTERSAT '95 exhibition in Frankfurt am
Main on 1995-02-03. Later I extended it with information taken from
DVB Project Office press releases and the DVB standards. I am not an
expert on the subject, only an interested observer who wants to make
some information about the future of digital TV available to the
interested network community.
2 Building blocks
The block diagram of a DVB receiver (aka "set-top box") looks roughly
like this:
cable or
antenna
|
V
receiver -> demodulation -> error correction -> optional access control
and decryption module
|
V
MPEG demultiplexer
|
+----------------------+-----------------------+
| | |
V V V
MPEG video decoder MPEG audio decoder data transmission
interface
| |
| | |
+---->-----+------. There
exists even a compatible extension to MPEG-2 that allows to encode 7+1
signals as they are used in good cinemas, which can be decoded as
either 7+1, 5+1, dolby surround, or stereo signals in the DVB
receiver.
[By the way: Originally, the U.S. digital TV standard "Grand Alliance"
planned to use the proprietary Dolby AC-3 encoding instead of standard
MPEG audio in their U.S. broadcasting system, which is not DVB
compatible but also based on MPEG-2. The results of their listening
tests showed that AC-3 provided superior quality to MPEG audio coding.
However, recently they discovered that the MPEG audio demo system used
in the "Grand Alliance" tests contained an implementation bug and new
listening tests showed now that MPEG audio and AC-3 produce fully
comparable signal quality. Let's hope that the U.S. abandons AC-3
quickly and therefore will not introduce their own incompatible audio
encoding system. This would make MPEG *the* single world-wide audio
and video encoding standard.]
The MPEG-2 standard also specifies higher levels (for high definition
TV) and higher profiles (which allow for instance to broadcast
different image quality layers with different error correction
overhead such that the image quality can degrade smoothly under bad
receiving conditions). As only an MPEG subset is used today, the DVB
system can easily be upgraded to these advanced functions later in an
upwards compatible way when the market is ready for HDTV. DVB systems
are required to skip data structures which they not not understand in
order to allow easy future backwards compatible extensions of the DVB
standards.
The bit rate utilized for a video program can be selected freely
depending on the quality requirements. Typical bit rates are:
2 Mbit/s: approx. VHS quality, suitable for simple signals
like comics
4-6 Mbit/s: approx. PAL quality (e.g., for talk shows)
8-9 Mbit/s: better than D2MAC and PAL+, comparable to studio
production quality (e.g., for cinema films and sport
events)
>15 Mbit/s: Various levels of HDTV quality
For the MPEG-2 audio encoding options, the DVB project has selected
the MPEG-1 layer II algorithm from ISO/IEC 13818-2 which is based on
the MUSICAM sub-band coding system. A typical audio bit rate is 192
kbit/s for a high quality stereo signal. MPEG allows arbitrary many
audio channels (for various languages or surround sound) to accompany
the video image.
The MPEG-2 standard also defines a multiplexing system (ISO/IEC
13818-1), which allows to combine many video, audio, and data streams
into one single data stream. In DVB, this multiplexing technique is
used to allow to put many different programs into for example one 38
Mbit/s data stream, which the DVB data transmission standards allow to
transfer per satellite transponder or per 8 MHz cable TV channel. This
38 Mbit/s bandwidth can be used to transmit simultaneously for example
either:
Video: each program 2 - 9 Mbit/s -> 18 - 4 video services
Audio: each program 192 kbit/s -> 190 radio stations
Data: each channel 64 kbit/s -> 590 data services
each channel 2.4 kbit/s -> 15800 data services
MPEG-2 multiplexing uses a packet length of 188 bytes, each packet
consists of a 1 byte header and 187 bytes data content.
SES expects that typically between six and eight programs will be
placed on one Astra transponder, which leaves between 4.75 and 6.34
Mbit/s per TV program. I would not expect much better image quality
than what you get with analog PAL from these bitrates. With four
programs per transponder (9.5 Mbit/s), MPEG-2 image quality should be
excellent.
2.1.1 Handling 16:9 images on 4:3 screens
The first cinema films were filmed in 4:3 aspect ration (the "academy
format"). Early TV standards adopted this format in order to stay
compatible with cinema. However, movie producers went on to the wider
16:9 format in order to to allow more impressive scenes (and perhaps
also to simply make cinema different from TV ;-). More recently, films
are often produced in the even wider 2.21:1 cinemascope format.
If a 16:9 or cinemascope image has to be displayed on a 4:3 screen,
there are two commonly used techniques to adapt the aspect ration of
the image: pan&scan and letterboxing.
Pan&scan means, that the broadcaster or movie producer has decided for
every part of the film, which 4:3=12:9 window of the full 16:9 image
is most interesting for the viewer. The remaining 25% image area will
be cut away on 4:3 screens:
+-----XXXXXXXXXXXXXXXXX--+
| X X |
| X X | X = visible 4:3 screen
| X X | - = full movie image
| X X |
| X X |
| X X |
+-----XXXXXXXXXXXXXXXXX--+
Pan & Scan Method
The second alternative is known as letterboxing. The size of the image
is reduced by 25% such that the full 16:9 image fits exactly onto the
4:3 screen with black bars on the top and bottom border of the image.
XXXXXXXXXXXXXXXXX
X X
X---------------X X = visible 4:3 screen
X X - = full movie image
X X
X---------------X
X X
XXXXXXXXXXXXXXXXX
Letterbox Method
Special 3/4 vertical filtering hardware is necessary which removes
every fourth line of the 16:9 image in a smooth way, in order to
provide high quality 16:9->4:3 letterboxing.
There are also compromise solutions possible:
XXXXXXXXXXXXXXXXX
+--X---------------X+
| X X| X = visible 4:3 screen
| X X| - = full movie image
| X X|
| X X|
+--X---------------X+
XXXXXXXXXXXXXXXXX
Combination of Pan&Scan and Letterboxing
The average viewer seems to prefer pan&scan and considers letterboxing
as a waste of valuable screen area. The cinema freak however knows
that pan & scan throws away 25% of the movie and that the 4:3 format
is not what the director and camera operator had in mind when they
produced the movie. Therefore, the expert viewer usually prefers
letterboxing.
It has so far been impossible for broadcasters to make both normal
viewers and cinema freaks happy at the same time. This changes with
digital TV! The movie is broadcasted digitally in whatever format of
the original film had. Pan&scan vectors are provided in the MPEG-2
data stream. They the tell the MPEG-2 video decoder which would be a
suitable place for the 4:3 pan/scan window on the wide movie image.
Whether pan&scan or letterboxing or some compromise format is used to
display the movie on a 4:3 screen is now decided in the DVB receiver
and not any more at the broadcaster. This allows every viewer
individually to switch between pan&scan, letterboxing, or some
compromise format.
Pan&scan or letterboxing is most important on 4:3 screens which are
very different from 16:9 and 2.21:1 cinema formats. If a 2.21:1
cinemascope movie is broadcasted, even if you have a 16:9 screen, also
some (much less serious) form of pan&scan or letterboxing has to be
applied in order to display the 2.21:1 format on your 16:9 screen.
2.2 Modem standards
As there have been no suitable international standards for digital
satellite, cable, and terrestrial modem transmission available before,
the DVB project had to define new standards here.
The various transmission mediums have very different characteristics,
therefore it is not possible to use only one single modulation
standard for all of them.
2.2.1 Satellite transmission (DVB-S)
Channel properties of satellite transmission are:
- relatively low signal/noise ratio
- large bandwidths available
- amplifier tubes in the satellites are operated in
a very non-linear mode for maximum power efficiency
The selected satellite modulation system is a 2 bit/symbol QPSK
(quaternary phase shift keying) modulation.
This modulation system is defined in the DVB-S standard ETS 300421.
Some characteristics are (this is only for interested modem experts):
- scrambling applied for spectral formation
- outer FEC: Reed Solomon Forward Error Correction RS(204, 188, T=8)
- convolutional interleaving (Forney)
- inner FEC: convolutional Code
- 35% half-Nyquist filtering
- flexible symbol rate
The DVB-S standard has a number of encoding parameters that allow to
choose an appropriate set of transmission parameters for the
characteristics of a specific satellite transponder. The reception
parameters that the DVB-S receiver must be able to handle are
- carrier frequency (GHz)
- polarization (horizontal, vertical)
- convolutional error correction code (1/2, 2/3, 3/4, 5/6, or 7/8)
- symbol rate (Mbaud)
The variable symbol rate allows the broadcaster to adjust the modem
exactly to the bandwidth available by the transponder. The selected
symbol rate will typically be the -3 dB bandwidth of the transponder
divided by 1.28. ETS 300 421 does not restrict the symbol rate in any
way.
The five different forward error correction (FEC) codes allow to vary
the amount of redundant information that is added to the MPEG-2
transport data stream. This way, the modem can be adapted for
different signal/noise ratios. For example, it might be possible that
a broadcaster who usually uses the 5/6 code goes back to the more
robust 2/3 code during an important football match when many people
might start to complain about reception problems caused by heavy rain
in their area. 3/4 means for example that 3 out of 4 bits carry MPEG
data, the remaining 25% of the bits are inserted for error correction.
The available different error correction codes allow to adopt the
modem transmission to the hydrometrological conditions in the
reception area. The broadcaster can select with the error correction
code how many expected minutes of image loss per year caused by heavy
rain are acceptable for the average viewer in the reception area.
The DVB-S system is optimized for single carrier per transponder Time
Devision Multiplex (TDM), however it is able to be used for
multi-carrier Frequency Division Multiplex (FDM) type applications.
There will be applications for satellite news gathering (SNG) where a
8 MHz channel is used in Single Channel Per Carrier (SCPC) technology.
SCPC means, there is only one program (with one video and its related
audio streams) transmitted on that carrier. This allows several DVB-S
uplink sites to share one single transponder by using different
carrier frequencies (FDM). The contrary to this is MCPC (Multiple
channels per carrier), which is used for normal broadcasts. All these
methods are compatible to (or at least can be performed compatible to)
the DVB-S standard.
2.2.2 Cable transmission (DVB-C)
Typical cable channel properties are:
- good signal/noise ratio
- small usable frequency spectrum
- echos and non-linear distortion
The selected cable modulation system is a 64-QAM (quadrature amplitude
modulation system with 64 symbols), which has the following
characteristics:
- high spectral efficiency
- low required bandwidth (8 MHz)
- requires adaptive pre-emphasis
This modulation system DVB-C is defined in ETS 300429 and some of the
technical details are:
- scrambling applied for spectral formation
- outer FEC: Reed Solomon FEC RS(204, 188, T=8)
- convolutional interleaving
- 15% half-Nyquist filtering
- optional 16-QAM and 32-QAM, extensible to 128-QAM or 256 QAM.
- flexible symbol rate
RS forward error correction and interleaving are identical to DVB-S,
which reduces the implementation effort in systems that convert DVB-S
into DVB-C.
The reception parameters that the DVB-C receiver must be able to
handle are
- carrier frequency (MHz)
- symbol rate (Mbaud)
- QAM type (64, 32, or 16 different symbols)
2.2.3 Community antenna installations (DVB-CS)
Several options are available for systems where one single satellite
dish supplies a large number of receivers:
System A: The head-end demodulates DVB-S signal and remodulates it
into DVB-C, so that normal DVB-C cable receivers can be connected to
the distribution network. This system is called SMATV-DTM (digital
transmodulation). Because of the high price for the additional modems,
this method is more interesting for very large cable TV installations.
Whether the head-end implements outer error correction or simply
passes data through without decoding and reencoding Reed-Solomon and
convolutional interleaving is optional (this does not cause
compatibility differences anyway).
System B: The cheapest solution for short cables: head-end converts
received signal only down to intermediate frequency above 950 MHz
without touching the QPSK modulation (SMATV-IF). A slightly more
expensive solution for longer cables and more receivers (SMATV-S): the
head-end converts the received signal down to the intermediate
frequency and then again down to a part of the VHF/UHF band like the
extended S-band (230-470 MHz) without touching the QPSK modulation.
2.2.4 Terrestrial transmission (DVB-T)
Channel properties of terrestial transmission are:
- very variable signal-to-noise ratio
- large-scale multi-path effects (reflections from nearby
house walls, etc. attenuate certain frequencies)
- very overcrowded frequency spectrum, interference from
nearby analog TV channels and sometimes ver far away
stations on the same frequency band.
The selected DVB-T modulation scheme has the following
characteristics:
- OFDM (Orthogonal Frequency Devision Multiplex)
In this technique, a fast fourier transform is used to
generate a broadcast signal with thousands of mutually
orthogonal QAM modulated carriers. A single symbol carries
therefore several kilobits of information. A guard interval
between symbols allows echos to pass by before the receiver
starts the detection of the next symbol.
- 8192 or 2048 carrier frequencies, each modulated using a
QAM with between 4 and 64 Symbols
- 8 MHz bandwidth
- outer FEC: Reed Solomon FEC RS(204, 188, T=8)
- outer convolutional interleaving
- inner FEC: Convolutional Code (1/2, ..., 7/8)
- inner interleaving
With thousands of separate carrier frequencies, the typical
elimination of some frequencies due to multi-way path reflections in
terrestrial transmission becomes tolerable as the interleaving and FEC
spreads the payload information uniformly across the entire bandwidth.
OFDM modulation is in particular also well suited for mobile and
indoor reception, as well as the operation of single-frequency
networks (SFN). In a single frequency network, many neigbouring
transmitters over a large area broadcast the exact same signal
carefully synchronized. The resiliance of the modulation scheme
against echos can handle this as merely an extreme multi-path
propagation as well, resulting in much better use of the frequency
spectrum as safety distances between transmitters operating on the
same frequency become less critical.
2.2.5 Compatibility
All three transmission systems are designed for maximal compatibility,
which means that they can use common circuit blocks (e.g., the Reed
Solomon decoder and interleaver) if a single receiver supports several
transmission mediums. Compatibility also means that transmodulation is
made easy when the bitrates are selected carefully.
As a practical example, let's have a look at the parameters used in
the Astra 1E satellite and the cable systems fed by it:
Many DVB-S satellite transponders (e.g., Astra 1E/1F/1G, DFS, Eutelsat
Hotbird) have a bandwidth of 33 MHz, which allows with QPSK a symbol
rate of 33 MHz / 1.2 = 27.5 Mbaud. With 2 bit/symbol, this results in
55 Mbit/s and after the convolutional 3/4 FEC decoder has removed 25%
of the bits for inner error correction, 41.25 Mbit/s remain. This bit
stream is sent to the second error correction algorithm
(Reed-Solomon), which transforms 204 bytes into 188 corrected bytes
and the final error corrected data rate is therefore 38.015 Mbit/s for
the multiplexed MPEG-2 data stream.
In a typical DVB-C cable system, there are 8 MHz channels, and after
the 15% roll-off specified by DVB-C, a theoretical maximum symbol rate
of 6.96 Mbaud is possible. For compatibility with the above DVB-S
example, let's use 6.875 Mbaud with a 64-QAM, which results again in
41.25 Mbit/s including Reed Solomon redundancy. As before, after the
RS-decoder, the final bit rate remaining is 38.015 Mbit/s.
As both satellite and cable modem parameters can be selected to use
the same final bit rate, satellite signals can easily be converted
into a cable signal by pure transmodulation. A satellite->cable
converter does not have to touch any Reed-Solomon codes or any MPEG-2
multiplex information. This allows cheap large community antenna
installations (see DVB-CS).
There are also Astra transponders with 26 MHz bandwidth and Eutelsat
plans for the W24 satellites 72 MHz bandwidth transponders. For these
satellites, other DVB-S symbol rates than 27.5 Mbaud will have to be
applied in order to utilize the full bandwidth of the respective
transponder.
The required worst case bit error probability for all modulation and
forward error correction (FEC) systems is 1e-11, i.e. there must not
be more than one wrong bit in the 38 Mbit/s MPEG data stream every 45
minutes, in other words not more than one wrong bit in a 8 Mbit/s TV
program every 3.5 hours.
2.3 System information and electronic program guide (DVB-SI)
DVB defines a Service Information standard (ETS 300468), which uses an
MPEG data channel in order to transmit technical parameters of the
transmission as well as electronic program guide (EPG) information for
the user.
Technical parameters transmitted include the broadcast parameters
(frequency, polarization, FEC, symbol rate, QAM type, MPEG multiplex
identifier, etc.) and identification of the services available in the
multiplexed channels. This allows the receiver software to configure
itself completely automatically, even if broadcast parameters change
(e.g., because of a satellite transponder rearrangement). The
satellite receiver must only be configured once to find the first
transponder. After a few minutes, the receiver will know the
parameters of all channels on all other transponders.
The Electronic Program Guide information transmitted includes:
a) Distribution network id (e.g. "SES Astra" or "German Telekom")
b) Channel id (e.g. "Sky Movies Gold" or "ZDF")
c) Program (e.g. "James Bond: Goldfinger")
d) Program type (e.g. "film", "news", "sports event", ...)
e) Service Provider (e.g. "British Sky Broadcasting")
f) Bouquet (e.g. "Sky Multichannels")
g) Broadcast time (e.g. 20:15 - 22:30)
h) Description text (e.g. "Classical secret service adventure,
actors: ..., director: ..., etc.")
All this information is not only transmitted for the current program,
but also for the following program, for alternate channels and for the
program of the next 2 weeks. All electronic program guide times are
transmitted in Universal Time (UTC) and it is the job of the software
in the receiver to translate it into the user's local time. This way,
the electronic program guide times displayed all over the continent
will all show the respective local time.
The Electronic Program Guide also contains the information necessary
to start/stop a VCR precisely with a film independent on any
unscheduled delays or commercial breaks in the program, and it
contains parental guidance information that allows parents to restrict
their children's access to on-screen sex and violence. DVB-SI
therefore also performs the functions of the VPS (Video Programming
System) that was available for these purposes on analog channels in
Germany and other countries since the early 1980s.
2.4 Teletext
The DVB-TXT standard (ETS 300 472) defines, how EBU-Teletext, which is
known for analog video services, can be transmitted within a
DVB-bitstream. This has been set up, because many of the now existing
analog programs will want to simulcast analog and digital signals in
order to make a migration from one system to the other easier. The
digital receiver is supposed to insert the teletext data into the
vertical blanking interval. A connected TV-set with built-in
EBU-teletext decoder can then handle this like the usual teletext.
DVB-TXT is not the specification of a new next generation teletext
service.
2.5 Subtitling
They say it's a standard for subtitling but it is very powerful and
can display many things, not just subtitles. It can be used as a very
high level teletext platform.
2.6 Conditional access control (pay-TV encryption)
Like the earlier pay-TV system EuroCrypt and VideoCrypt, DVB access
control will be separated in two parts: decryption and descrambling.
Descrambling operates directly on the MPEG transport data stream or on
individual channels located inside this stream. The DVB project has
standardized a Common Scrambling algorithm. It is a combination of a
64-bit block cipher followed by a stream cipher algorithm. The
technical details of the Common Scrambling algorithm are secret and
only made available under a Non-Disclosure-Agreement for access
control hardware manufacturers.
The decryption is the section of the access control system that
receives reception entitlement messages from the MPEG transport
stream, decrypts and processes them, manages the subscription
activation status of this receiver and finally forwards to the
descrambling part the control word necessary to descramble the next
few seconds of the program. Unfortunately, the DVB consortium has not
been able to standardize a common encryption algorithm. One of the
primary reason was the fear of pay-TV piracy (e.g., cloned smart cards
with the copied decryption keys and algorithms).
Two alternative solutions have been discussed that would still allow
to use one single receiver in order to receive channels from different
pay-TV providers: The Common Interface and SimulCrypt.
2.6.1 Common Interface (DVB-CI)
The Common Interface is a slot for one or even several type 1 or type
2 PCMCIA cards (now called PC-Card). The receiver can send the full
MPEG-2 transport data stream as it comes out of the demodulator and
error correction units through the card plugged into the Common
Interface, before it will be processed by the MPEG demultiplexer in
the DVB receiver. If several CI cards are present, the MPEG transport
data stream will be passed sequentially through all these cards.
A CI card can request control over decoder functions. It can talk to
ISO 7816 smartcard slots which the receiver might also provide in
order to keep parts of the decryption algorithm there, it can generate
on-screen messages and receive remote control commands, and it can
talk to a telephone modem which might be present in the receiver in
order to handle online pay-per-view and near-video-on-demand
functions.
A CI card will typically contain a hardware implementation of the
Common Scrambling algorithm as well as a security processor with
decryption software. Parts of the decryption software might also be
implemented in an ISO 7816 smartcard that can be exchanged at much
lower cost in case of successful piracy hacks against the decryption
system (just as this has happened several times already with
VideoCrypt and EuroCrypt).
After power-up, a Common Interface card acts like a PCMCIA memory
card. Several memory locations act as data transmission registers that
allow communication with the processor on the CI card. This way, a CI
card can also be plugged into a normal laptop PCMCIA slot and some
limited card functions could be accessed this way. When the
descrambler in the CI card is activated, the most-significant-byte of
both the address and data bus turn into 8-bit parallel input and
output busses for the MPEG transport data stream. Other pins will then
be used as separate byte clock and packet clock lines for both
directions.
Although the Common Interface was primarily intended to allow plug-in
decryption modules, it is not limited to that application. May be,
this interface will in the future be used to connect other extension
modules to the DVB receiver, like Internet browsers, interactive TV,
PC video boards for harddisk recording, high speed PC satellite data
acquisition systems, etc.
2.6.2 SimulCrypt
The problem of the non-existence of a common decryption standard can
be solved by plugging CI cards for all required decryption standards
into the receiver.
The existing standard for the Common Scrambling algorithm allows
however also another approach: A broadcaster might provide the
decryption data for its channel in the formats necessary for various
decryption systems in the data stream. This way, various different
encryption system would decrypt the entitlement messages with their
proprietary methods, however finally they would all find the same
control word which they send to the same Common Scrambling algorithm.
The SimulCrypt idea is not new: BSkyB has been using it for years on
those channels like MTV on Astra that can be received both by
VideoCrypt 1 and VideoCrypt 2 receivers simultaneously.
SimulCrypt is a nice way of avoiding the need to have several CI cards
plugged into one receiver. It is therefore cheaper and more user
friendly. However, SimulCrypt works only if the broadcaster from which
you bought your decryption system cooperates with the pay-TV
broadcaster from which you want to see a channel. For full broadcaster
independence, you will still need the Common Interface in your
decoder.
3 Market overview and system introduction schedule
German Telekom has installed in >150 German CATV networks hardware for
three digital 8 MHz channels which will reach 80% of all German cable
users (11.5 million households). Astra 1E, the first satellite of the
European Astra series designed completely for DVB-S compatible digital
broadcast has been launched in late 1995 and is currently already
broadcasting several DVB channels. DVB-S consumer receiving equipment
for Astra 1E and other DVB-S satellites will start to be available in
1996, when the Astra 1E test phase ends. The second DVB-S satellite
Astra 1F has also been launched recently. Eutelsat on 13 degrees east
is also broadcasting a number of DVB-S channels.
In the far future, the German Telekom CATV system might perhaps use
all channels for digital transmission, which gives a total bit rate of
>2 Gbit/s. This would allow to transmit for instance 237 highest image
quality TV programs (8 Mbit/s each) or 950 medium quality TV signals
(2 Mbit/s each) or a combination of various quality levels somewhere
between.
Although DVB is a European system, it was used for the first time
outside Europe (Australia and South Africa). Within Europe, we have in
some countries very clear constellations and in others very unclear
ones. In the UK there's ol' Rupert Murdoch and BSkyB and nobody else
who might launch digital satellite services. Since Sky has a lot of
analog transponder capacity, they don't seem to hurry towards DVB.
But, there is a strong momentum towards DVB-T with 2K carriers (other
broadcasters than Sky).
In France, the major player is of course Canal+. They plan to start a
regular service spring 1996. The largest TV-market in Europe, Germany,
lies down paralyzed by the silly fight of two very big companies:
Kirch and Bertelsmann/MMBG. Both of them want to introduce their own
decoders, which are, in spite of the existence of standards, NOT
compatible. Thus, the market is splitted in two and a potential
customer has to decide between two boxes. With one box you can watch
the Kirch package, with the other box Bertelsmann's. If you want both
packages - buy two boxes. The advantage of Kirch is his immense stock
on movies and licenses. Bertelsmann is now the third largest media
company in the world. Recently, they have joined forces with Rupert
the Australian, mainly to overcome the problem with the lack of movie
licenses. So there is a fight between two systems just like VHS versus
Beta for home video. My personal estimation is, that services will not
start before the end of this summer.
Another big fish is Nethold, planning services in Italy, the Benelux
countries and Scandinavia.
4 Hints for DVB receiver buyers
So what reception equipment is available? Up to now, there's only
expensive professional equipment available. Most of these devices have
still some software bugs and other teething troubles. Some of them
only work with a certain kind of encoder so in some cases you can
tell, what encoder they have on the transmitter side. It's all still
in a stadium of experimenting. Consumer equipment (set top boxes) will
become available together with the launch of services. The closest
estimation one can make: this year.
Here are a few things on which you might want to pay attention to when
you decide to buy a DVB receiver. Keep in mind that especially the
first generation will have a number of limitations like
- not all modem parameters are supported (which might limit you to
reception from a specific satellite type or broadcaster)
- the DVB-CI common interface is not yet supported (which might
limit you to a single pay-TV provider)
- the subtitling standard is not yet supported
Especially the common interface (CI) is a very important feature in
case you are interested in pay-TV. I highly recommend not to buy a DVB
receiver that is not equipped with a CI slot (which will cause around
around 80 DEM additional manufacturing costs). As there will be no
common single DVB encryption standard, by buying a DVB receiver with
only a built-in fixed decryption module, you will be bound to the
offerings of one single pay-TV provider. With the CI slot, you will be
able to easily exchange the complete encryption module yourself and
your DVB receiver will not get obsolete very quickly. Unfortunately,
the DVB-CI standard has not yet been finished, so it will take quite
some time until the first DVB receivers with common interface become
available. When the DVB-CI standard is ready, ask specifically for the
common interface slot when you buy your receiver! Better DVB receivers
might even provide several common interface slots such that you don't
have to switch PCMCIA modules when you switch channels.
Good DVB receivers will display the current bit error rate of the
signal, which will help you to position the dish very accurately in
order to avoid image dropouts caused by bad weather. An optional audio
signal indicating the signal strength is also very helpful when you
are sitting on the roof aligning your dish onto the satellite.
Good DVB receivers should be able to display the MPEG-2 parameters
(bitrate, resolution, etc.) of the current channel in some technical
information submenu so that you can get an objective information about
which image quality the broadcaster has decided to be good enough for
you (especially important if you are technically interested and pay
for the quality you get!). MPEG-2 theoretically allows to bring full
studio quality to the home, but the actual image quality depends on
the bitrate assigned to a channel which depends on the number of
channels per transponder. When managers decide which image quality is
sufficient to their viewers, the result might be something around 4
Mbit/s which is not necessarily any better than current analog PAL,
while a critical eye would prefer twice the data rate for a really
good image quality.
Although a DVB receiver will be able to configure itself completely
automatically, it should still offer you to easily rearrange the
allocation of the channels on the channel selection menus and the
remote control channel numbers.
In a good DVB receiver you can easily change the initial channel
parameters (frequency, polarization, FEC, symbol rate, etc.), so that
you can get the electronic program guide information not only from one
fixed transponder on one satellite, but can use other future DVB
satellite systems as they emerge. You should also be able to enter the
reception parameters manually in order to be able to have a look at
those channels which are not mentioned in any electronic program
guide.
Good DVB receivers will not only provide pan&scan display of 16:9 and
2.21:1 movies on your old 4:3 TV set, but also allow you to switch to
a letterboxing presentation or to select a combination of pan&scan and
letterboxing.
As MPEG-2 can optionally encode not only stereo but also 5+1 surround
sound, you might want to check whether the DVB receiver you plan to
buy is supporting 5+1 surround sound decoding.
If you own a video cassette recorder with VPS support or a TV set with
VPS parental guidance password lock, then you should check, whether
the DVB receiver you are interested in can automatically reproduce the
VPS signal from the Electronic Program Guide information, so that your
VPS video recorder still works.
If you want to use a community antenna installation, make sure your
DVB receiver supports the DVB-CS SMATV alternative that your community
antenna is using (see the section on DVB-CS): For SMATV-DTM, you will
need a DVB-C cable receiver, for SMATV-IF or SMATV-S you'll need a
DVB-S receiver that has an IF or S-band receiver, respectively.
Good DVB receivers will allow over-the-air updates of their
microcontroller software, which is important especially for first
generation devices that usually have a few software bugs and
compatibility problems.
5 Literature and sources for more information
The DVB-related standards and further information are available from:
DVB Project Office
European Broadcasting Union
Case postale 67 - 17A Ancienne Route
CH-1218 Grand Saconnex (Geneva)
Switzerland
phone: +41 22 717 27 19
fax: +41 22 717 27 27
e-mail: dvb@pax.eunet.ch
Web: http://www.ebu.ch/dvb_home.html
International Organization
for Standardization (ISO)
Case Postale 56
CH-1211 Geneve 20
Switzerland
phone: +41 22 749 01 11
fax: +41 22 733 34 30
e-mail: sales@isocs.iso.ch
Web: http://www.iso.ch/
European Telecommunication
Standards Institute (ETSI)
F-06921 Sophia Antipolis CEDEX
France
phone: +33 92 94 42 00
fax: +33 93 65 47 16
Web: http://www.etsi.org/
http://www.etsi.org/technicalactiv/dvb/dvb.htm
In particular the ETSI web site is worth a look as it has now all the
DVB standards available for free download (registration necessary).
Quite good information about the DVB market situation and new rumors
and developments can be found in the monthly German sat-TV magazine
"infosat" (ISSN 0933-6907).
A German book that covers in detail the modem and compression
technology behind the DVB and MPEG standards is
Ulrich Reimers: Digitale Fernsehtechnik -- Datenkompression und
Übertragung für DVB, Springer-Verlag, Berlin, 1995, 291 pages,
ISBN 3-540-58993-7, DEM 98,-
Goetz Kluge mentioned that there is
now a 2nd revised edition: Ulrich Reimers: DVB, 1996, ISBN
3-540-60945-8. An English translation might be available at the end of
this year.
An English book about DVB is
Hervé Benoit: Digital Television : MPEG-1, MPEG-2, and Principles of
the DVB System, Arnold, London, 1997, ISBN 0-470-23810-4.
A very good source of information on DVB for people in North America is
http://www.coolstf.com/mpeg/
Copyright (C) Markus Kuhn 1996.
[end]